Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
  • G01N30/02—Column chromatography
  • G01N30/84—Preparation of the fraction to be distributed
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D15/00—Separating processes involving the treatment of liquids with solid sorbents; Apparatus therefor
  • B01D15/08—Selective adsorption, e.g. chromatography
  • B01D15/26—Selective adsorption, e.g. chromatography characterised by the separation mechanism
  • B01D15/40—Selective adsorption, e.g. chromatography characterised by the separation mechanism using supercritical fluid as mobile phase or eluent
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
  • G01N30/02—Column chromatography
  • G01N30/84—Preparation of the fraction to be distributed
  • G01N2030/8429—Preparation of the fraction to be distributed adding modificating material
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
  • G01N30/02—Column chromatography
  • G01N30/04—Preparation or injection of sample to be analysed
  • G01N30/06—Preparation
  • G01N30/10—Preparation using a splitter
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
  • G01N30/02—Column chromatography
  • G01N30/62—Detectors specially adapted therefor
  • G01N30/72—Mass spectrometers
  • G01N30/7233—Mass spectrometers interfaced to liquid or supercritical fluid chromatograph
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T137/00—Fluid handling
  • Y10T137/8593—Systems
  • Y10T137/87249—Multiple inlet with multiple outlet

Definitions

• This invention relates to flow splitters for use in fluidic systems including chemical instrumentation systems such as chromatographic, extraction and reaction systems.
• flow splitters are used to present some representative portion of the output of a flow system to an electronic detector, typically to measure the progress of some chemical processing step [e.g. separation, extraction or reaction progress].
• Analytical liquid chromatography is used for separation of complex mixtures of solutes dissolved in a liquid solvent.
• a high pressure pumping system is used to force a liquid mobile phase through a separation column packed with very small, high surface area silica spheres that have been chemically modified to interact at differing degrees with different types of solutes.
• a strong interaction between the modified silica surface and a solute will cause the solute to be retained at the surface for a longer period than a weak interaction.
• the liquid mobile phase also plays a significant role in the time a particular solute spends on the separation column.
• Weak solvents are those that are unable to remove solutes from the separation surface. Strong solvents are those that readily redissolve solutes that are even strongly adsorbed on the surface.
• a technique called gradient elution is used. In this case, the original mobile phase starts at a composition that makes it a very weak solvent. Gradually, the composition is changed by increasing the relative concentration of strong solvent. The process continues until a maximum composition is reached or all solutes have been eluted from the column.
• solutes introduced to a separation column emerge as individual solutes at different times and in different flow segments of the mobile phase based on the strength of their interaction with the modified silica surface and the mobile phase.
• the flow can be directed through an electronic detector which can sense presence of specific types of solutes based on the solutes' physical properties [e.g. molecular mass, UV- visible light absorption, refractive index, electrochemical reduction or oxidation potential, etc.].
• Such detectors generate an electronic signal that in analytical chromatography is simply used to quantitate the amount of solute present, or qualify the presence or absence of a particular component in the mixture. Because of the greatly improved performance of using high pressure pumps through packed columns of very small particles, the modern technique has become known high performance liquid chromatography or HPLC.
• Preparative high performance liquid chromatography extends the art of analytical liquid chromatography by adding a collection step to the instrument system.
• the electronic signals generated by one or more detectors are used to trigger the collection of specific segments of the mobile phase flowstream that contain the desired solutes.
• a purification is performed which competes economically with other purification methods such as recrystallization.
• preparative HPLC systems must be scaled up to significantly higher flow rates, solute concentrations and column sizes in order to process significant amounts of material.
• solute concentrations used in preparative HPLC are disproportionately high compared to analytical HPLC.
• a typical analytical HPLC separation may apply 5 microliters ( ⁇ l) of sample mixture to the analytical separation column in a mobile phase flowstream of 1 mL/min.
• the solute concentrations of the mixture are typically in the range of 100 ⁇ g/mL, so a total of 500 nanograms are applied into the system.
• Preparative SFC systems require a high pressure backpressure regulator to maintain the CO2 at liquid-like densities that keep it soluble with organic liquids and solutes.
• the backpressure setting ranges from 100 bar to 300 bar.
• the mobile phase of SFC is subject to phase change, including up to a 500-fold volume expansion and significant temperature drop as the pressure goes from 100 bar to atmospheric pressure as it passes through traditional restrictor-based flow splitters prior to entry into the detector.
• phase change including up to a 500-fold volume expansion and significant temperature drop as the pressure goes from 100 bar to atmospheric pressure as it passes through traditional restrictor-based flow splitters prior to entry into the detector.
• This localized change of phase, flow, temperature and viscosity within the splitter restrictor makes the split behavior unpredictable, especially over the range of compositions and concentrations found during gradient chromatography.
• the low diameter tubing required is subject to plugging, especially when the evaporating CO2 carries with it some of the organic solvent solvating the solutes in the mobile phase. This evaporative solvent loss coupled with the intensly cold temperatures can easily result in precipitation of solutes from the remaining organic solvent in the tubing.
• Construction of a useful flow splitter must take into account the physical properties of the mobile phase that is to be sampled. Such properties may include, for example, the flow rate, viscosity, phase (e.g. gas, liquid, supercritical), pressure, or composition variation (change in concentration of solvent, change in solutes eluted) over time. Each of these elements may affect the performance of the flow splitter. In addition, the flow and concentration requirements of the target detector or detectors must be taken into account. What is needed is a flow- splitting device that can reliably control the split ratio from a primary flowstream from moderate to very high split ratios while diluting sample concentrations to levels appropriate to the target detectors.
• the device must overcome the problems associated with fixed splitters including the effects of pressure and viscosity variations and phase change in the region where the actual flow splitting occurs.
• the device must be able to operate in a continuous manner at maximum pressures higher than 100 bar, for example up to about 300 bar, and eventually deliver the split flow to an atmospheric pressure device.
• the device should be suitable for both HPLC and SFC type mobile phases.
• the present invention provides a splitter for a pressurized primary flowstream of a mobile phase.
• the splitter comprises:
• a first splitting stage that divides the primary flowstream between a major split flowstream and a minor split flowstream by means of a first and a second restrictive flow element
• a first dilution stage that combines a dilution flow source with the minor split flowstream to create a diluted minor flowstream
• a second splitting stage that divides the diluted minor flowstream between a secondary major diluted flowstream and a secondary minor diluted flowstream by means of a third restrictive element in the secondary minor diluted flow stream;
• a conditioning flow source conditions the secondary minor diluted flow stream after the third restrictive element prior to outlet of the secondary minor diluted flowstream from the splitter.
• the present invention provides a splitter for a pressurized primary flowstream of a mobile phase, the splitter comprising:
• a first splitting stage that divides the primary flowstream between a major split flowstream and a minor split flowstream by means of a first and a second restrictive flow element
• a first dilution stage that combines a dilution flow source with the minor split flowstream to create a diluted minor flowstream
• a second splitting stage that divides the diluted minor flowstream between a secondary major diluted flowstream and a secondary minor diluted flowstream by means of a third restrictive element in the secondary minor diluted flow stream;
• a conditioning flow source conditions the secondary minor diluted flow stream after the third restrictive element prior to outlet of the secondary minor diluted flowstream from the splitter;
• Figure 1 is a diagram of an embodiment of the invention with a pressure balanced first splitting stage, a first dilution stage, a second pressure controlled splitting stage, and a second dilution stage;
• Figure 2 is a diagram of another embodiment of the invention without pressure balancing.
• Figure 3 is a diagram of another embodiment of the invention with multiple serial splitting and dilution stages.
• the present invention is directed to a high split ratio flow splitter that can handle the flow characteristics of typical HPLC mobile phases and pressures as well as the extended range of pressures, viscosities and phase transitions associated with supercritical fluid chromatography mobile phases.
• flow splitters of the present invention can handle main flow rates from 1 mL/min to 1000 mL/min.
• Conventional single-stage flow splitters achieve splits up to about 20,000:1 for a preparatory flow rate of 50-100 mL/min., whereas splitters of the present invention can achieve split ratios of well over 1,000,000: 1.
• the mobile phase in supercritical fluid chromatography may experience a change in state over the course of the chromatography due to variations in temperature and/or pressure through the system.
• a CO2 mobile phase may exist as a highly compressed and liquified gas, a supercritical fluid, or as a low viscosity, compressible fluid at different stages of the preparation.
• the term "supercritical fluid” will refer to those compounds that do exist or can exist in the supercritical fluid state during chromatography, regardless of the actual state of the compound at any particular point in time during chromatography and collection.
• other compounds useful in SFC include, but are not limited to, ethane, propane, nitrous oxide, butane, isobutene, sulfur hexafluoride, water, hydrochlorofluorocarbons, hydrofluorocarbons, alkanes, or any combination of these.
• the mobile phase in SFC optionally includes an organic liquid solvent.
• Suitable organic solvents include, for example, lower alcohols such ethanol, methanol, 2- propanol, isopropanol, n-butanol as well as a variety of less polar solvents such as acetonitrile, dioxane, methylene chloride and the like .
• the mobile phase in standard HPLC implementations using the flow splitter of the present invention may comprise, for example, water, aqueous buffers, acetonitrile, and alcohols for the most common use of this technique.
• the present invention is a passive restrictor network comprised of one or more alternating stages of flow splitting and dilution that occur serially in the split flowstream. Preferably, there are at least two or more flow splitting and dilution stages. Together, the serial stages reduce the total absolute contribution and relative concentration of solutes from the main flow stream that are transferred to the final electronic detector.
• the term "passive" in connection with restrictor means that the restrictor is not electronically controlled.
• Tunability of the splitter is achieved hydraulically, by increasing or decreasing one or more dilution flows into the splitter to change both dilution and split ratio values.
• Pressure balancing within the system in addition to a backpressure regulator, dramatically attenuates variation of the splitter performance in the presence of significant changes in pressure, viscosity and density and flow rate in the mobile phase.
• effective split ratios for 100 mL/min flow systems can range from 1000: 1 to well over 2,000,000: 1, while effective dilution of the main stream concentration can range from 50: 1 to over 10,000: 1. Desired split ratios for this flow rate are approximately 200,000: 1 with dilution > 1000: 1. Since the flow rate for preparative SFC can range from 2 mL/min. to 2,000 mL/min., the split ratios will vary depending on the flow rate.
• Splitters of the present invention can operate with primary flow rates of a minimum of 2 mL/min., 10 mL/min. or 12.5 mL/min., anywhere up to flow rates of 800 mL/min., 1,000 mL/min., or 2,000 mL/min., depending on the needs of the user.
• the main flow which exits the separation column is first split by a balanced flow restrictor pair into major and minor flowstreams.
• the major and minor flowstreams are eventually rejoined at the exit of the first splitter. This insures that the downstream pressure regulation will affect both restrictors of the splitter equally.
• a minimum pressure of about 100 bar is maintained at the outlet of the first splitting stage to insure that SFC mobile phase components remain at their liquid-like densities.
• addition of a dilution flow in at least 90% excess to the minor split flow of the first splitting stage creates a flowstream that has essentially the same physical properties of the dilution solvent and is relatively independent of the properties of the main flow stream mobile phase components.
• the composition of the diluted split flowstream remains virtually constant in these parameters. This permits all subsequent splitting stages of the invention to be performed using traditional pressure-based, resistive flow splitting techniques.
• the dilution flow rate will vary depending on the main flow rate. For example, for a 100 mL/min. main flow rate, a desirable dilution flow rate would be from 0.5 to 20 mL/min., more preferably between 0.5 to 5 mL/min. A minimum flow rate must be maintained to prevent back flow from the major flowstream into the minor flowstream via the pressure balancing tee of the the system. The maximum dilution flow rate can be up to 20% of the main flow rate.
• FIG. 1 An embodiment of the invention is displayed in Figure 1.
• two splitting and dilution stages, 60 and 62 are used to reduce the fractional contribution of the main flowstream and the absolute concentration of solutes that enter split detector 96.
• Main flow source 64 enters a first splitting and dilution stage 60 which is comprised of splitting tee 66; restrictive flow elements 68 and 70; mixing tee 72; low restriction flow conduits 74 and 76 and pressure balancing tee 78.
• splitting tee 66 restrictive flow elements 68 and 70
• mixing tee 72 mixing tee 72
• low restriction flow conduits 74 and 76 low restriction flow conduits 74 and 76 and pressure balancing tee 78.
• Dilution flow source 80 provides additional solvent flow to the minor flowstream at mixing tee 72 and this combined flowstream passes in majority through conduits 74 and 76 to pressure balancing tee 78 where it recombines with the major flowstream and exits the splitter.
• Main flow continues through a variety of optional flow elements such as time delay element 82 and in-line detector 84 to eventually reach backpressure regulator (BPR) 86, which maintains the upstream flow at an elevated pressure.
• BPR backpressure regulator
• Pressure balancing tee 78 of the first splitting and dilution stage 60 has a major effect on the performance of the splitting function.
• the pressure drop across restrictors 68 and 70 becomes a function only of the pressure drop across the splitter stage itself and not the absolute pressure of the flowstream.
• a relatively small pressure drop of 3 to 30 bars across the restrictors can be achieved, even in the case where the backpressure regulator is elevating the flowstream to hundreds of bars of absolute pressure. This feature is particularly important in the case of SFC mobile phases, since pressure tee 78 maintains the outlet end of split restrictor 70 at the high pressure controlled by the BPR.
• pressure balancing the first splitter minimizes the effect of the physical properties of the mobile phase on the split ratio.
• flow restrictors 68 and 70 respond similarly in pressure drop to changes in viscosity, density or flow rate, the split ratio remains constant, since both branches of the split flow experience the same change at the same time.
• An example of such behavior is found in a pair of laminar flow tubular restrictors, which are governed by the Hagen- Poiseuille relationship in equation 1 :
• the split ratio for splitter 60 is calculated as the ratio of flows though the major and minor flow restrictors 68 and 70 respectively as shown in equation 3 where constant terms have been represented by k.
• Equation 3 reduces to final two ratios of tubing radius and length.
• the radius ratio is to the fourth power so a small change in radius will have a dramatic on the relative flows.
• the invention does not require that restrictors 68 and 70 be of the same type.
• a laminar flow capillary tube might be the best solution.
• another type of restrictor may be employed. The only requirement is that the restrictors vary in ⁇ p in generally the same manner with changes in the composition of the mobile phase.
• a laminar flow restrictor may be matched with a restrictor of any type that increases in pressure drop in a generally linear manner with viscosity.
• a porous bed restrictor which varies according to Darcy's Law shown in equation 4,
• this flow will be ⁇ 500 ⁇ L/min to allow a minimum 10: 1 dilution of the split flow with a 5 mL/min dilution flow source such as a standard analytical pump.
• suitable restrictors include orifice restrictors and active resistors.
• the pressure balancing tee 78 provides a low restriction path for excess dilution flow from source 80. As higher flow rates are delivered to mixing tee 72, the dilution ratio increases. If conduits 74 and 76 are sized properly to cause minimal pressure drop, the dilution ratio may be changed over a very wide range without affecting the split flow from restrictor 70 or downstream split flows discussed below. This provides the first splitter stage with the capability of adjusting the dilution ratio by simply increasing or decreasing dilution flow from source 80. Such adjustability provides a facile way of tuning the splitter to handle a much wider range of analyte concentrations in the main flowstream.
• Second splitting and dilution stage 62 which overlaps with first splitting and dilution stage 60, is comprised of inlet conduit tube 74; splitting tee 88; flow conduit 76; restrictive flow element 90; and mixing tee 92.
• This second splitting stage operates very differently from the primary splitting stage described above.
• the fluid in inlet tube 74 is the result of a significant dilution of the original mobile phase composition. So long as this dilution is at least 10: 1, the composition of fluid contained in conduit 74 is essentially constant, with physical properties very closely resembling the dilution solvent despite any changes arising in the main mobile phase composition.
• dilution ratios of 10: 1 to 200: 1 are desirable in this stage, more preferably 10: 1 to 50: 1, for a 5 mL/min. flow rate, which makes the contribution of the main mobile phase even lower.
• Splitting tee 88 divides the flow into two streams.
• the major flowstream is typically conduit 76 which rejoins the main flowstream at pressure balancing tee 78 and communicates the pressure from backpressure regulator 86 back to splitting tee 88. It is the pressure at splitting tee 88 that determines the flow through restrictor 90, and thereby the split ratio of this stage.
• restrictor 90 delivers flow to mixing tee 92 at nearly atmospheric pressure.
• the ⁇ p across restrictor 90 will be the same as the gauge pressure measured at pressure balancing tee 78. Further, this pressure will typically be determined by the setting of BPR 86. For HPLC systems this value may be up to a few hundred psi.
• the value will typically be in excess of 100 bar [1450 psi] up to 300 bar, to maintain the supercritical components at a liquid-like density.
• the pressure is maintained between 100-200 bar.
• restrictor 90 must be sized based on prior knowledge of the backpressure range to be used.
• flow through restrictor 90 should be limited to allow sufficient additional dilution within the flow range of the split detector. It should be noted that the effects of phase transition of supercritical components are very minor at this point due to the high dilution of the prior step. Gaseous components will typically remain dissolved within the organic liquid at ratios of 50: 1 or greater under even under these conditions. Since the composition of fluid entering splitter tee 88 is relatively constant, the minor split flow through restrictor 90 can be calculated from equation 1.
• Flow exiting restrictor 90 enters mixing tee 92 where it combines with conditioning flow from source 94.
• the conditioning fluid may contain chemical reagents such as buffers to induce ionization of the target solutes in solution in order to achieve a particular type or level of ionization in split detector 96.
• Flow from conditioning flow source 94 will also add further dilution to the detector flowstream.
• the total flowrate that leaves mixing tee 92 is limited by the flow requirements of split detector 96. As an example, it is common for mass spectrometer detectors to limit flow to 1 mL/min for certain common ionization probes to perform properly.
• Both the dilution flow source and the conditioning flow source are tuned (as to flow rate and composition) by means of an electronic controller (not shown).
• splitters that uses more than one stage of splitting and dilution can be easily assembled without the same issues.
• a first stage a 2,000: 1 split and 100: 1 dilution is achieved.
• a second stage of splitting and dilution is performed on the split flow stream with 100: 1 split ratio and 20: 1 dilution.
• the effective split ratio from the two stages is 200,000: 1.
• the effective dilution ratio of the two dilution steps is 2000: 1.
• each split restrictor, 70 and 90 would carry 50 ⁇ L /min flow. Since the volume of restrictors 70 and 90 typically is much than 1 ⁇ L, the timing delay of this process can be maintained at under 2 seconds which compares favorably to the very difficult, multi-second process of creating a single split flow of 0.5 ⁇ L/min [e.g. 200,000: 1 split ratio] diluted with 999.5 ⁇ L/min of dilution solvent.
• Restrictor flow rates used in the consecutive stages of splitting and dilution approach are 100 times larger than single stage splitting flows and offer continuous washing of the flow system to avoid significant peak distortions due to poorly swept flow regions.
• a 100 cm length of 500 ⁇ m id tubing is used as restrictor 68 and a 35 cm length of 75 ⁇ m id tubing is used for restrictor 70.
• the split ratio would be calculated from equation 3 as 691: 1, with 72 ⁇ L/min passing through restrictor 70 and the remaining flow passing through restrictor 68.
• the calculated pressure drop across this first splitting stage would range from approximately 80 psi [5.5 bar] for pure water to 30 psi [2.1 bar] for pure acetonitrile which are the typical HPLC mobile phase solvent components.
• the dilution ratio of the minor flowstream for this stage assuming addition of 5 mL/min from dilution flow source 80, would be 70: 1.
• a selection of a 20 cm length of 40 ⁇ m id tubing for restrictor 90 allows a flow of approximately 34 ⁇ L/min. This produces a split ratio of 151: 1 for the second splitting stage.
• Addition of a final conditioning flow of 0.966 mL/min gives a secondary dilution of -30: 1.
• Final determination of split and dilution ratios is achieved by calculating the product of sequential split or dilution steps. From the example given, the final split ratio would be the product of 691 for the first stage and 151 for the second split stage for a total split ratio of - 104,000: 1. Thus less than 0.001% of the original injected sample enters the split detector. For a 100 mg sample this would be less than 1 ⁇ g delivered to the detector. Similarly, serial dilutions of 70 and 30 fold produce a final dilution of -2100: 1 or -0.05% of the solute concentration. These conditions allow a solute of 10 mg/mL emerging from the separation column to enter the split detector at less than 5 ⁇ g/mL concentration.
• the conditioned sample is delivered to split detector within 0.1-5 seconds of entering the splitter, more preferably within 0.1-2 seconds. Such short delays at this high dilution are not possible using conventional single stage splitting and dilution techniques without severe distortion of peak shape.
• restrictor 68 is typically selected to provide a pressure drop of between 30-120 bar for the highest viscosity flow which is approximately 0.3 cP at 50% methanol composition.
• selection of restrictor 70 as a 30 ⁇ m id by 20 cm long capillary tube allows 60 ⁇ l/min flow or a split ratio of 838: 1.
• the dilution ratio is 85:1.
• Pressure balancing tee 78 set at a typical level of 100 bar or higher insures no outgassing of supercritical components during this splitting stage.
• the second stage in this system follows equation 1 for flow through restrictor 90. Hence a restrictor of 25 ⁇ m id and 25 cm length will provide -41 ⁇ l/min flow of methanol when the BPR is set to 100 bar.
• the split ratio of this stage is 123: 1.
• Addition of 1 mL/min conditioning flow further dilutes the second split flow by 52: 1.
• the total split and dilution ratios can be calculated to be 103,000: 1 and 2150: 1 respectively, which is very similar to the HPLC case.
• the split detector Due to the very high dilution of the splitter in each case, the split detector receives virtually identical compositions from each system (HPLC and SFC) and should get identical results. Thus the splitter also provides the benefit of making the measurement independent of original flow source. This is not the case for traditional SFC vs. HPLC splitting devices measured by mass spec, where differences in ionization and suppression levels are frequently found.
• the embodiment of the splitter shown in Figure 1 is suitable for incorporation into any preparative chromatograph control system.
• signals from a mass spectrometer in the split detector position and from a UV detector in an in-line position can be used to signal the presence of a solute in the main flowstream just before it reaches the collection system.
• the time delay element 82 may be placed either before or after in-line detector 84 depending on the time required by the controller to respond to the trigger signals and initiate collection. Timing delays should be minimized between the time solute from the main flowstream exits the splitter at tee 78 and the time the conditioned split flow reaches the split detector. This prevents the need for long delays in the flow stream which can degrade the separation efficiency of the system.
• a general target of less than 5 seconds delay here is sufficient for typical operation, but performance can be improved further with shorter delay times, for example between 0.1 to 2 seconds.
• FIG. 2 An additional embodiment of the invention is disclosed in Figure 2. Like Figure 1, the embodiment has two split and dilution stages 100 and 102 respectively to serially dilute a portion of the main flowstream for split detector 96. The significant difference between these embodiments is the lack in Figure 2 of the pressure balancing tee 78 shown in Figure 1. Removal of the pressure balancing tee eliminates the automatic compensation in the first splitter stage 100 for changes in flow, viscosity and pressure. As a result, this stage will vary more widely in performance over the course of a standard gradient separation common to HPLC and SFC systems. Further, these factors (flow, viscosity and pressure) must be manually calculated in the design of the splitter and will limit its operation to a smaller dynamic range than the embodiment shown in Figure 1.
• splitting and dilution stage 100 is comprised of splitting tee 104; main flow conduit 106; split flow restrictor 108; and mixing tee 110.
• the splitter receives flow from main flow source 64 which divides into major and minor flowstreams flowing through conduit 106 and restrictor 108 respectively.
• Dilution source 80 supplies dilution solvent at mixing tee 110 to reduce the concentration of solutes in the flowstream.
• the diluted flow stream is passed to splitting and dilution stage 102 via conduit 112.
• Second splitting and dilution stage 102 is comprised of splitting tee 114; restrictors 116 and 118 and mixing tee 120. Flow enters this stage via conduit 112 and is split into major and minor flowstreams through restrictors 116 and 118 respectively. Restrictor 118 carries excess diluted flow to waste 122 or alternately to a second split detector in the flow system (not shown). Split flow from restrictor 116 is diluted at mixing tee 120 by conditioning flow 94, then delivered to primary split detector 96.
• restrictors In the case of SFC systems, restrictors must be selected to maintain a high enough backpressure on the output of splitting stage 100 to prevent outgassing of the supercritical components. This needs to be done along with predetermination of the minimum flow from the dilution source so that some range of dilution source tuning can be achieved.
• the splitter may also require the backpressure regulation of the system to be increased (as compared to the embodiment shown in Figure 1) for both HPLC and SFC cases to insure sufficient pressure drop across both stages.
• FIG. 3 displays an embodiment with three splitting and dilution phases.
• split and dilution stages 60 and 62 act as described in Figure 1.
• the minor stream of split and dilution stage 62 in this case delivers to third split and dilution stage 130 rather than directly to the split detector 96.
• Stage 130 is comprised of splitting tee 132, flow restrictors 134 and 136 and mixing tee 138.
• a waste receptacle 122 receives flow diverted through restrictor 136.
• splitting and dilution stage 130 requires a pressure drop across the restrictor network to operate. However, this may be quite small, on the order of a few bars, and should not interfere with significantly with the larger pressure drops in upstream stages. As a result, the first two stages experience little effect from the addition of the third stage.
• the dilution flow scheme of Figure 3 is also worthy of note.
• a single dilution source, 80 supplies dilution solvent to both stages 60 and 62. This is accomplished by addition of restrictor 140 which limits dilution flow to stage 62 based on the back pressure from BPR 86.
• the dilution flow received into the first stage will be total flow minus the flow through restrictor 140. Since both dilution flow and split flow through restrictor 90 are governed by BPR 86, they will vary proportionately with changes of the BPR. As a result, the dilution ratio will remain constant in this case.
• a separate flow source such as a pump may be used for dilution at stage 62 if more direct control is required.
• a third stage such as stage 130 would be implemented to allow more overall dilution of the main stream before reaching the detector.
• flow from a second dilution source such as conditioning flow 94 could be increased without regard of the split detector capacity to yield a higher dilution.
• the advantage of serial dilution over three steps also allows a far greater range and efficiency. For example three serial dilutions of 10 ⁇ l/min split flows to 1 mL/min total flow [e.g. 100: 1 dilutions] would yield a final 1,000,000: 1 final dilution while consuming less than 3 mL/min of dilution flow. This is substantially less solvent than would be required with one or even two stages of dilution.
• This example shows the extensibility of adding further splitting and dilution stages to the invention to achieve very high dilution ratios.
• Viscosity range for SFC is pure CO2 to pure methanol
• Viscosity range for HPLC is pure H20 to pure acetonitrile

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